Mollies Keep Having Babies? Here’s What to Do


Here’s another batch of Molly fry! You must be ecstatic… or not.

If having more and more fry is overstocking your tank and giving you trouble instead of joy, you need to learn how to reduce your fry or avoid it before it happens.

To avoid having too many Molly fry, you can:

  • Keep only females in the tank or separate your Mollies by gender
  • Make tank conditions unfavorable to breeding
  • Let natural food chain take its course and get fries eaten by adults
  • Sell or give away newborns

Before we discuss the methods listed above, it will help to find out first why Mollies seem to multiply exponentially in such a short time.

Why do Mollies keep having babies?

Reason #1: They’re livebearers

To experienced hobbyists, this statement alone explains itself. Just say you’re taking care of livebearers and they instantly know their breeding habits. Other species such as Guppies, Swordtails, Endlers, and Platies are some aquarium favorites that behave the same way.

Livebearers are certainly prolific breeders. It takes only 1 male to impregnate multiple females. Specifically, these lady Mollies can give birth every 30 days to up to 100 babies. Larger Mollies like the Sailfin gestate for about 60 days.

Livebearers have the advantage of being born a swimming fish in what is known in science as viviparity. It’s an advantage for fish as they can escape predators easier as opposed to being laid as eggs that are just sitting somewhere waiting to be hatched (and eaten). But it also translates to more surviving Molly fry in your community tank.

At the speed with which Mollies reproduce, they are classified as invasive.

Reason #2: Female Mollies can store sperm

And here’s where the mystery begins for a lot of beginner fish keepers. They wonder why their female Mollies get pregnant even without males around.

Female Mollies have this ability called sperm storage. They can get pregnant several times over a few months from only 1 male contact.

Mollies Keep Having Babies? Here’s What to Do
Mollies Keep Having Babies? Here’s What to Do

This explains why you bring some female adult Mollies home hoping to keep just them but end up with fry anyway. They’ve already had contact with males in the pet store and now bring you these surprises. Whether these surprises were pleasant or panic-inducing would dictate your next actions.

Before you complain to your pet store, just know that these behavior and characteristics are normal for Mollies.

How to prevent or stop Molly pregnancies:

  • Keep only one gender or separate males from females

Mollies are livebearers, meaning they are non-stop breeders. If kept together, you’ll keep having babies every 30 days with every pairing. If you have 5 females and 2 males in your school, that’s 5 batches of fry every month!

Separating male Mollies from female Mollies is crucial to stop pregnancies. But keep Reason #2 (above) in mind, as they may already be pregnant even after you separate the fish.

If you plan to restock, it helps to buy female Mollies that are still juveniles so they can’t get pregnant yet. Or you can buy from a single-gender tank that your pet store has kept since these fish were juveniles. Just remember that if you’re getting all males, get at least 5 to keep aggression down.

  • Keep the tank “unromantic”

Altering conditions in the tank to keep Mollies from mating is possible. It can also be done after separating the genders to discontinue pregnancies. Don’t go to extremes though, as there is a fine line between keeping them from breeding and stressing them to the point of illness or death.

To breed, Mollies need a temperature of about 78 to 80 degrees Fahrenheit. You can lower this to 71 to 72 degrees which they can thrive on.

You can also reduce their protein intake if they’re on a diet that consists of live food such as frozen or live bloodworms and brine shrimp. Replace them with flakes and algae wafers instead.

While it may help, discouraging Mollies from breeding is not a guarantee that they won’t breed.

What do you do when a Molly Fish has babies?

So the babies are here already, that’s fine. You can still lower the population with a few methods.

Mollies keep having babies
What do you do when a Molly Fish has babies?
  • Let nature run its course

Not doing anything has probably lowered the actual number of your fry. Adult Mollies are notorious for eating their little ones. If you want to encourage this type of behavior, reduce the number of times you feed your Mollies. If you feed them every day, you can skip a day of feeding twice a week so they’ll be more inclined to eat the fry if they haven’t polished them off yet.

You can also remove a few plants and decorations so the fry will have fewer places to hide in.

With this method, only a few fries will survive which is more manageable for you.

While we want to suggest that you keep other fish that eat Molly babies such as Angelfish or Gouramis, you might end up with fry of these fish instead.

  • Sell or give away the fry

A friend or two can help you out by taking some fry off of your hands if they’re interested. You can also join community fishkeeping groups in your area so you can swap or sell fish and other aquarium-related items. Not only do you get cheap and hard-to-find items but important knowledge from other people’s experience as well.

Giving or selling some fry to your pet store can also be an option. Some stores might not give you money but probably a discount on your next purchase. It’s still a win-win situation.

Things NOT to do

There are some things that you should avoid at all costs because of certain consequences.

Never release unwanted fish into the wild. Mollies are invasive species and will threaten local fish populations if released irresponsibly. Native species are vital to the ecology of an area and their elimination will disturb that balance.

It has already happened in certain places. For example, the Desert Pupfish, a local species in California, has become endangered ever since the Sailfin Molly took over the region. The same Molly species has also caused the decrease of populations of native Damselflies in Oahu, Hawaii.

Places like Vermont, California, New Jersey, Florida, New South Wales and Queensland in Australia, and many more places (check your local environmental laws) prohibit the release of non-native species into their bodies of water. You could be penalized or at least fined for doing so.

So don’t throw them to the nearest river or stream or flush them down your toilet. Fish might survive in the sewers and find themselves out in the wild.

Never stress out a pregnant Molly to the point that she dies. When we advise altering the conditions of your tank to become unfavorable to breeding, don’t do so to the point that you kill your fish.

Never starve fish to death. Mollies will eat fry more if you skip a feeding or two. But don’t skip for a whole week of feeding that they become too sick or start getting stressed with the lack of food. You won’t be dealing with just hunger but probably fights among the adults. This will snowball into other problems like treating injuries or infections and death.

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